A hardware-based fusion algorithm which employs 3-dimensional voxel volumes has been developed and tested for sensitivity and accuracy by fusing paired combinations of Magnetic Resonance Imaging (MRI-T1, MRI-T2), Positron Emission Tomography (PET) and x-ray Computed Tomography (CT) images. Current software allows for the simultaneous fusion of as many as 4 muti-modality image sets using the full organ volumes asregistration landmarks. The use of organ surfaces for the fusion landmarks results in an accuracy which is superior to that obtained by using the conventional method which employs three orthogonal 2-dimensional views. Current plans are to adapt this methodology for 4-dimensional treatment planning, using time-dependent image sets to improve target margins and minimize normal tisseu complications. The Electron-Gamma Shower (EGS-4)-based Monte Carlo Dose Calculation Engine (DCE) has been fully implemented in both a LINUX and Windows environment. In the Windows environment, the DCE has been integrated into a full featured treatment planning system. Work is now centered on the development of phase-space source models. The determination of each phase-space model requires substantial computing power and we are porting the code in a manner that will permit the use of parallel processing that, hopefully, will reduce the calculation time required to test model parameters from 3-4 weeks to under 1 day. Currently, this system is being used to investigate small field stereotactic radiosurgery. Due to the reduced field size, edge effects become important and the size of detectors used to access the radiation output affect the measurement results. Monte Carlo simulation of these output measurements will greatly assist in the selection of the detector system that is most suitable. In a related project, we are adapting both algebraic and Monte Carlo DCEs to predict organ doses received from diagnostic CT scans. The characterization of the x-ray beam from a GE CT Scanner used for clinical scanning at Children's National Medical Center is complete. We have also completed absolute dosimetry measurements linking the standard diagnostic measurement of Computer Tomography Dose Index (CTDI) to actual absorbed dose. We are now embarking on a series of verification measurements usilg Thermolumeniscent Dosimeters (TLDs) to confirm the model.